Fine particles such as oxide fine particles, nitride fine particles, and carbide fine particles have been used in the production of sintered bodies, for example, dielectric materials for semiconductor substrates, printed wiring boards and various, electrically insulated parts, materials for high-hardness and high-precision machining tools such as dies and bearings, functional materials for grain boundary capacitors, humidity sensors and the like, or precision sinter molding materials, and in the production of thermal sprayed parts, for example, engine valves, of materials that are required to be wear-resistant at a high temperature, as well as in the fields of electrode or electrolyte materials and various catalysts for fuel cells. Use of such fine particles improves bonding strengths between different ceramics or different metals in a sintered body or thermal sprayed part, or denseness or functionality thereof.
One of the methods for producing such fine particles is a vapor-phase method. The vapor-phase method includes a chemical method that involves chemically reacting various gases or the like at high temperatures and a physical method that involves applying an electron beam or laser beam to substances to decompose or evaporate the substances so as to form fine particles.
An example of the vapor-phase method is a thermal plasma method. The thermal plasma method is a method of producing fine particles by instantaneously evaporating a raw material in thermal plasma and then quenching and condensing/solidifying the evaporated material to produce fine particles. This method has many advantages such as high cleanness, high productivity, applicability to high melting point materials because of high heat capacity at high temperatures, and easy preparation of composite material particles as compared with other vapor-phase methods. Therefore, the thermal plasma method is often used as a method of producing fine particles (see, for instance, Patent Document 1).
In the conventional process for producing fine particles using a thermal plasma method, fine particles are produced by powdering a raw material, dispersing the powdered material (powder material, or powder) with a carrier gas to charge the material directly into a plasma flame (see, for instance, Patent Document 1).
Patent Document 1 describes a technique of producing fine particles by introducing (supplying) powdered raw materials into a thermal plasma flame to evaporate the materials and quenching the resulting vapor-phase mixture, that is to say, a method of producing oxide-coated fine metal particles which involves combining powder materials for fine metal particles and for a coating layer with each other, introducing a raw material mixture into a thermal plasma (i.e., thermal plasma flame) in an inert or reducing atmosphere so as to evaporate the raw materials to obtain a vapor-phase mixture, and quenching the mixture thus obtained.
In the method of producing fine particles described in Patent Document 1 as above, the vapor-phase mixture is cooled by separating the mixture together with a plasma gas, a carrier gas, and a gas derived from a powdery raw material far enough from the thermal plasma flame to introduce the mixture into a quenching tube for cooling it. It is also described in Patent Document 1 that the vapor-phase mixture is cooled in the course of separation far enough from the thermal plasma flame by an intermediate cooling section provided upstream of the above-mentioned quenching tube.
Patent Document 1: JP 2000-219901 A
However, the method of producing fine particles described in Patent Document 1 uses the technique of directly introducing a powdered raw material into a thermal plasma flame, in which the powdered raw material tends to agglomerate and is difficult to make monodisperse, so complete reaction of the raw material in the thermal plasma flame cannot occur, thus giving adverse influence on the uniformity of fine particles or resulting in a decrease in quality such as generation of impurities. Also, in the case where a raw material is in the form of powder, it is difficult to continue to introduce or supply accurately a fixed amount of raw material into a thermal plasma flame, so the resultant fine particles tend to become non-uniform.
In addition, in the conventional cooling technique described in Patent Document 1 as above, it is difficult to uniformly cool a vapor-phase mixture, and hence the fine particles formed tend to have a non-uniform particle size or shape. Also, the fine particles just after the formation tend to collide with one another to cause agglomeration, which gives an adverse influence on the uniformity of the particle size and shape of the fine particles. Further, the cooling performance of the above-mentioned cooling technique depends on the amounts of a plasma gas, a carrier gas, and a gas derived from a powdery raw material, and it is difficult to keep the gases constant in amount. Therefore, with this cooling technique, it has been difficult to control the particle size and the uniformity in particle size of the fine particles formed.
Further, since it is generally difficult to uniformly cool the vapor-phase mixture obtained by evaporation of raw materials, the formed fine particles tend to be non-uniform in shape or particle size, and the fine particles just after the formation tend to collide with one another to agglomerate, which gives an adverse influence on the uniformity of the fine particles.